We plan to address two fundamental questions about cerebral cortical circuits: " What are the functional roles of different neuronal classes, pyramidal cells and subclasses of inhibitory interneurons, in cortical computations? " What are the properties of spontaneously active ensembles and how do these ensembles relate to the neurons'functional properties? We will use two-photon calcium imaging of the rodent visual cortex in vivo to measure the orientation selectivity of all neurons in a volume layer 2/3 (~500 5m on a side, ~10000 neurons). The unprecedented ability to record neural activity so completely will enhance our understanding of two aspects of cortical circuits: the diversity of cell classes and the richness of spontaneous activity. We will use anatomical and genetic techniques to identify pyramidal neurons and various classes of interneurons. Spontaneous activity will be assessed with two-photon calcium imaging in the absence of sensory input. Great advances have been made over the past several years on the molecular and cell- physiological classification of different neuronal subclasses. These advances have had little impact on in vivo studies of neural function, but now calcium imaging with single-cell resolution can show the functional correlates of classes of neurons that have been identified by genetic techniques. Spontaneous activity in the cerebral cortex has been well documented, but it is poorly understood. Simultaneous cellular-level monitoring of entire neural circuits is crucial for advancing the study of this correlated activity and its relation to visual function. A wide-field resonant-scanning system will be used so that we can image thousands of neurons in a three-dimensional volume simultaneously. This large-scale overview of the function of entire local circuits will enhance our understanding of cerebral cortical function in health and disease. This basic research on cortical sub-networks is directly applicable to clinical disorders of brain function. The neurological and psychiatric diseases with the largest impact on public health, Alzheimer's disease, stroke, epilepsy, depression and schizophrenia, are all disorders of cortical activity. The approaches outlined in this proposal will characterize cortical circuits with unprecedented completeness;they can be used to study both normal brains and models of neurological disease.